A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Generally, electromagnetic radiation that emanates from a radiation source is focused by a collector to converge the radiation when entering an illumination system for illuminating the patterning device. The illumination system typically includes several optical elements, such as mirrors to illuminate the patterning device using the radiation beam conditioned by the illumination system.
In order to limit the amount of the radiation beam's illumination power that is lost due to reflections generated by the mirrors of the illumination system, it has been proposed to limit the number of mirrors present in the illumination system.
Limiting the amount of mirrors may decrease the ability to predetermine the direction from which the radiation beam emanating from the radiation source enters the illumination system.
For a significant number of possible designs of the illumination system, it is desirable to have the radiation beam be incident from below the illumination system. It has, therefore, been proposed to transmit the radiation beam through a floor supporting the illumination system, but the floor would need to be significantly modified in order for the radiation beam to be able to pass through the floor.